As integrated circuit memory devices become more highly integrated, the demand for high performance gate electrodes increases. In particular, as memory capacities of dynamic random access memories (DRAM) exceed 1 gigabit, gate electrodes made from materials having a low resistivity and a work function corresponding to half the energy gap of silicon are needed. Currently, polycide structures including a silicide (a heat-treated compound of metal and silicon) which is formed on polysilicon are used to form gate electrodes. In particular, tungsten silicide and titanium silicide are widely used in these polycide structures.
As gate electrode materials improve, the resistance thereof decreases, and memory devices using these improved gate electrode materials can perform operations more quickly. Other problems may, however, result. For example, a titanium-polycide gate electrode can have a resistivity which is about one quarter that of a comparable tungsten-polycide gate electrode. When a titanium-polycide gate electrode is etched, however, the sidewalls of the gate electrode pattern may be eroded significantly. This erosion problem may be reduced by using low temperature test processes or time modulation systems. These approaches, however, may reduce process margins.
Platinum-polysilicon gate electrode structures are currently being studied to address the above mentioned problems. In particular, a platinum-polysilicon gate electrode structure can be used to reduce the gate electrode resistivity, to reduce erosion of gate electrode sidewalls, and to reduce the complexity of the gate electrode structure. For example, the resistances of various materials used to form comparable gate electrode structures are as follows: the resistance of a tungsten polycide gate can be approximately 80 .mu..OMEGA..multidot.cm; the resistance of a titanium polycide gate can be approximately 20 .mu..OMEGA..multidot.cm; and the resistance of a platinum polysilicon gate can be approximately 10 .mu..OMEGA..multidot.cm.
Platinum, however, is so chemically stable that it does not readily produce compounds having significantly high vapor pressures. Accordingly, platinum may be difficult to etch. When a platinum layer is etched using a photoresist mask and a chlorine series plasma gas, the photoresist may be etched more quickly than the platinum film. Accordingly, a photoresist mask may be insufficient when etching platinum making it difficult to form patterns having relatively high resolutions. The etching selectivity with respect to the photoresist is higher when a fluorine-series plasma gas is used to etch platinum. After etching a platinum layer using a fluorine-series gas, however, significant polymeric residues may be produced on the sidewalls of the patterned platinum layer.
Furthermore, when a fluorine-series plasma gas is used to etch a platinum layer, oxide may be etched too quickly to act as a mask. When a chlorine-series plasma gas is used, an oxide mask may be damaged. An oxide mask should thus be five times thicker than the platinum layer being patterned. Accordingly, an oxide mask may generate micro loading problems during the formation of patterns having relatively high resolutions. In addition, an adhesion layer may be needed to bond the platinum layer and the oxide layer which is used to form the oxide mask.